The mammalian nervous system is composed of a multitude of distinct neuronal subtypes, each with its own phenotype and differential sensitivity to degenerative disease. Although specific neuronal types can be isolated from rodent embryos or engineered from stem cells for translational studies, transcription factor mediated reprogramming might provide a more direct route to their generation.
The mammalian central nervous system (CNS) is assembled from a diverse collection of neurons, each with its own unique properties. These discrete characteristics underlie the proper integration and function of each neuron within the circuitry of the brain and spinal cord. However, their individual qualities also render particular neurons either resistant or sensitive to particular degenerative stimuli. Thus, for each neurodegenerative disease, a stereotyped set of neuronal subtypes is destroyed, causing the hallmark presentation of that condition. Therefore, if one is to comprehend the mechanisms that underlie the development, function and degeneration of the CNS, it is important to first deeply understand the properties of individual neuronal subtypes.
Physiological and biochemical studies of individual neuronal types have been greatly facilitated by the ability to isolate distinct classes of neurons and interrogate them in vitro. Most studies have focused on neurons isolated from the developing rodent CNS. However, it is not routinely possible to isolate analogous populations of human neurons or to isolate and fully study differentiated central neurons. Pluripotent stem cells, such as embryonic stem cells (ESCs), may provide an inexhaustible reservoir of diverse neural subtypes, offering an attractive approach for in vitro studies (Wichterle et al., 2002). Although stem cells have shown great promise, to date, only a handful of neural subtypes have been produced in this way. Furthermore, in many cases the neuronal populations produced from stem cells have not been shown to possess refined subtype specific properties and may only superficially resemble their counterparts from the CNS (Peljto and Wichterle, 2011).
Experiments using the reprogramming of one set of differentiated cells directly into another suggest an alternative approach for the generation of precisely defined neural subtypes. Using distinct sets of transcription factors, it is possible to reprogram fibroblasts into pluripotent stem cells (Takahashi and Yamanaka, 2006), blood progenitors (Szabo et al., 2010), cardiomyocytes (Ieda et al., 2010) as well as functional, post-mitotic neurons (Caiazzo et al.; Pfisterer et al., 2011; Vierbuchen et al., 2010). Thus, it may be possible to use factors which act on cells intrinsically, rather than relying on morphogens that act extrinsically to more precisely specify the exact properties of a wide array of neuronal types. Most reprogramming studies have so far only produced induced neurons (iNs) with an unknown developmental ontogeny and a generic phenotype (Pang et al., 2011; Pfisterer et al., 2011; Vierbuchen et al., 2010). Recently, two studies have generated cells that resemble dopaminergic neurons based on the production of tyrosine hydroxylase (Caiazzo et al.; Pfisterer et al., 2011). However, it is unclear whether these cells are molecularly and functionally equivalent to embryo- or ESC-derived dopaminergic neurons. In particular, it has yet to be determined whether any type of neuron made by reprogramming can survive and properly integrate into the CNS. If neuronal reprogramming is to be successfully applied to the study of CNS function or degeneration, then it must be capable of producing specific neuronal types that possess the correct phenotypic properties both in vitro and in vivo.
Motor neurons control the contraction of muscle fibers actuating movement. Damage to motor neurons caused by either injury or disease can result in paralysis or death; consequently, there is significant interest in understanding how motor neurons regenerate after nerve injury and why they are selective targets of degeneration in diseases such as spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS). There is a need to produce functional motor neurons for the treatment of motor neuron degenerative diseases and disorders.